Liquid developers generally include a liquid phase, comprising an insulating carrier liquid such as an isoparaffinic hydrocarbon, and a solid phase, comprising toner particles composed of a pigment and a binder. The solid phase toner is dispersed or suspended in the liquid phase carrier. Liquid developers further include a minor amount of charge director which insures that the toner particles are uniformly charged with the same polarity, which may be either positive or negative depending upon the particular application. The liquid developer is used to develop a latent image formed on a photoconductive imaging surface. Usually the photoconductive surface is charged with one polarity; and the toner particles are charged with an opposite polarity. If the liquid developer contains excessive charge director, then the images developed will tend to be somewhat faint because of loss of image charge due to leakage in the higher conductivity liquid developer. On the other hand if the liquid developer contains insufficient charge director, then the images developed will also tend to be somcwhat faint since toner particles having reduced charge move with reduced velocity through the developer liquid to the imaging surface.
A more serious problem with liquid developers having insufficient charge director is that the toner tends to drop out of suspension, forming sludge deposits which continually grow until operation of the electrostatic copier must be interrupted for cleaning. It is the maintenance of the charge on the toner particles by the charge director which causes the toner particles to repel one another, maintaining them dispersed, and preventing them from agglomerating and forming sludge deposits